Engine Exhaust

Natural gas-fired engines and turbines are often used to run compressors, generators, and pumps throughout the natural gas value chain. In many operations, part of the natural gas stream is used to power these engines and turbines. Unlike other emission sources that primarily emit methane, engine and turbines emit large quantities of conventional air pollutants and only a very small fraction of methane. Methane emissions result from incomplete combustion, fugitive emissions from the fuel gas supply, or engine malfunctions.

Internal combustion engines and turbines are used in the natural gas industry to power compressors to move natural gas, to power pumps to move liquids, to drive generators to provide electricity, and to operate mechanical machinery, such as pump jacks and drilling rigs. Most engines and turbines are fired on natural gas and vary in size from engines as small as 10 horsepower (hp) used to operate pump jacks at an individual well up to engines or turbines of several thousand hp used to move natural gas through interstate transmission pipelines.  

Natural gas-fired turbines are used throughout the midstream segment of the gas industry to drive electrical generators, and to power compressors. Small gas turbines or “microturbines” (< 300 hp) can be used to drive electrical generators that provide primary or backup power to some oil and gas operations, such as offshore platforms, gathering stations and natural gas processing plants. Larger turbines are used to power centrifugal natural gas compressors to move natural gas through gathering pipelines and through interstate gas transmission pipelines. Turbines used in the gas industry are typically simple cycle turbines that range in size from 300 to 20,000 hp. 

• Reciprocating engines use pistons that move back and forth to convert pressure into rotating motion and are typically used in the natural gas industry to generate electricity. At pipeline compressor stations, these engines are used to power reciprocating compressors that move natural gas down the pipeline from station to station. These engines have different configurations, two-stroke or four-stroke. Two-stroke engines are simpler, have fewer moving parts than four-stroke engines, have reduced maintenance, and increased reliability compared to four-stroke engines, but have higher emissions. The combustion process in these engines is classified as either lean burn or rich burn, which operate with different air-to-fuel ratios and combustion efficiencies that impact the formation of different pollutants. Rich burn engines generally have lower methane emissions.     
• Turbines operate with rotary motion and are composed of three major components: compressor, combustor, and power turbine. Gas-fired turbine driven centrifugal compressors have significantly lower emissions of all pollutants and methane than reciprocating engines and can carry two to three times the capacity of engines. However, they are more sophisticated in operation and, therefore, not used in remote, unmanned applications such as well heads and most small gathering/boosting stations. One turbine powered centrifugal compressor can replace three reciprocating engine compressors at a processing plant or gas transmission station. Turbine driven centrifugal compressors are not as flexible for start/stop operations, fuel gas quality, or changing load as engine-driven reciprocating compressors. 

The combustion process in the engines and turbines result in a number of air pollutants, including nitrogen oxides (NO_x), sulfur oxides (SO_x), carbon monoxide (CO), volatile organic compounds (VOC), and methane in small proportions. Nitrogen oxides are formed from the reaction of oxygen with either atmospheric or fuel-bound nitrogen during the combustion process. Sulfur oxides are formed from the reaction of oxygen with hydrogen sulfide (H₂S) and mercaptan sulfur in fuel gas. Methane emissions occur as a result of incomplete combustion of the natural gas fuel. These methane emissions are also sometimes referred to as “combustion slip.” Methane emissions also result from leaks in the fuel gas supply line to the engine or during engine malfunctions.

U.S. Environmental Protection Agency. (2000, April). Compilation of air pollutant emission factors, Volume I: Stationary point and area sources (AP-42), Fifth Edition, Section 3.1 (Stationary gas turbine). https://www.epa.gov/sites/default/files/2020-10/documents/c03s01.pdf

U.S. Environmental Protection Agency. (2000, August). Compilation of air pollutant emission factors, Volume I: Stationary point and area sources (AP-42), Fifth Edition, Section 3.2 (Natural gas-fired reciprocating engines). https://www.epa.gov/sites/default/files/2020-10/documents/c03s02.pdf

Vaughan, T. L., Luck, B., Williams, L., Marchese, A. J., & Zimmerle, D. (2021). Methane exhaust measurements at gathering compressor stations in the United States. Environ. Sci. Technol., 55, 2, 1190-1196. https://doi.org/10.1021/acs.est.0c05492 (now https://pubs.acs.org/doi/10.1021/acs.est.0c05492)

Zimmerle, D. J., Williams, L. L., Vaughn, T. L., Quinn, C., Subramanian, R., Duggan, G. P., Willson, B., Opsomer, J. D., Marchese, A. J., Martinez, D. M., & Robinson, A. L. (2015). Methane emissions from the natural gas transmission and storage system in the United States. Environ. Sci. Technol., 49, 15, 9374-9383. https://doi.org/10.1021/acs.est.5b01669  (now https://pubs.acs.org/doi/10.1021/acs.est.5b01669)